This invention relates to liquid crystal display devices incorporating display cells of the type with picture elements (pels) arranged in a matrix array, and in which the liquid crystal layer is sandwiched between an electroded transparent front sheet and a rear sheet formed by or carrying a semiconductive layer provided with access circuitry by which the display is addressed on a line-by-line basis via a matrix array of semiconductor gates directly or indirectly connected with an overlying matrix array of liquid crystal cell electrode pads.
For alphanumeric data displays fabricated over silicon matrices the dyed cholesteric nematic phase change effect provides good contrast for a given brightness of ambient lighting. The phase change effect exhibits a significant measure of hysteresis in the characteristic relating extinction coefficient to drive voltage, but this can be tolerated in binary level display type alphanumeric displays whose pels are either `fully on` or `totally off`. Clearly the hysteresis presents problems in the display of data involving multilevel grey scale representation.
A solution to this problem is to use a dyed nematic mixture with a limited amount of chiral content providing between 90.degree. and 360.degree. twist of the director through the thickness of the liquid crystal layer. This does not provide as much extinction as the dyed phase change mode, but has the advantage that the hysteresis is substantially eliminated.
Calculations using CIE lightness scales and some subjective experimentation indicate that some improvement in subjective perception of contrast with only slight loss in brightness can be achieved by partial additional absorption of the weakly absorbed plane of polarisation of light. Detailed studies (e.g. Judd & Sysecki: Colour in Business, Science and Industry pp 264-271, 2nd Edition, 1963, published by John Wiley & Sons Inc.) indicate that the perception of grey levels is very dependent upon the brightness of the background to the display and upon the state of adaptation of the eye. Hence it would be useful to be able to control the extinction of the pels in part in accordance with the information to be displayed and in part in accordance with the perceived ambient lighting conditions.